Discrete-time radio frequency (RF) is a newly emerging field in wireless digital communications wherein analog RF signals that are transmitted over-the-air are directly sampled into a discrete-time sample stream suitable for digital signal processing. A typical wireless digital communications device would use analog filters, duplexers, mixers, analog-to-digital converters (ADC), etc. to convert the analog RF signals into a digital data stream that is suitable for digital signal processing. Unfortunately, analog circuit components, especially components such as capacitors, inductors, resistors, etc., necessary for the analog filters are difficult to integrate into an integrated circuit. This is especially true for the precise values of these components required for use in filters. Of course, it is the desire of the manufacturer to maximize the degree of integration for the wireless transceivers (or receiver). This is because the more highly integrated a wireless transceiver can become, the lower the production costs for the transceiver and the transceiver will typically use less power during operation.
Discrete-time RF involves the direct conversion of the analog RF signal into discrete-time sample stream through the use of a direct sampling mixer, without having to undergo any intermediate analog filtering, downconversion, etc. An example of a direct RF sampling mixer is one that uses current to perform its sampling. The current-mode direct sampling mixer converts the received analog RF signal into a current that is then integrated by a sampling capacitor. The charge on the sampling capacitor is then periodically read to produce the discrete-time sample stream.
After reading the charge on the sampling capacitor, it is common practice to reset the charge on the sampling capacitor back to zero and/or to place a bias voltage value on the sampling capacitor. A reason for doing so is to prevent the accumulation of charge during the sampling phase to saturate (or deplete) the charge storage capacity of the sampling capacitor, thus resulting in loss of information. For example, should there be a relatively large charge already on the sampling capacitor when it begins to accumulate charge during the integrating phase, it is possible for the accumulated charge when combined with the existing charge to be above the maximum (or be below the minimum) amount of charge that may be stored on the sampling capacitor. Once the sampling capacitor becomes saturated or depleted, information is lost.
One commonly used technique for resetting the sampling capacitor is to short the sampling capacitor to electrical ground to remove any charge from the sampling capacitor and then applying a known current to the sampling capacitor for a known amount of time. This develops a known voltage onto the sampling capacitor. The known voltage is commonly referred to as a bias voltage and the operation of placing the bias voltage onto the sampling capacitor is known as a precharge operation.
A disadvantage of this technique is the amount of power consumed in bringing the sampling capacitor up to the bias voltage value. Each time that the sampling capacitor is brought from zero volts to the bias voltage value, a significant amount of current is consumed. If the precharge operation occurred only infrequently, then it is possible to overlook the power consumption. However, the precharge operation occurs after each sample is read out from the sampling capacitor and depending on the sampling rate, the precharge operation can occur very frequently. This leads to the consumption of a considerable amount of power. In many applications, such as portable and battery powered radios, power consumption is of vital concern and should be minimized when possible.
A need has therefore arisen for a method and apparatus to bring the sampling capacitor to a prespecified bias voltage value with minimum power consumption.